Now that the FAA issued an emergency AD to address fatigue cracking in some 175 Boeing 737 Classics, the question arises: how could have Boeing so wildly miscalculated the interval at which inspections of this particular area of fuselage should occur?

It might takes months before anyone knows for sure, but it seems clear that the April 1 incident in which a five-foot gash opened in a Southwest Airlines 737-300 stands to heighten concerns about the scores of narrowbody and regional jets that have become the workhorses of the air transport industry.

Of course, engineers expect high-cycle operating environments to place extra strain on airplane skins due to repeated pressurization and depressurization. However, the incident appears to have surprised Boeing. Previously, its inspection guidelines called for such checks at 60,000 cycles. After the Southwest 737 developed the tear in its fuselage, Boeing cut that interval in half because the 15-year-old airplane had accumulated only 39,781 flight cycles and 48,740 flight hours.

The AD calls for eddy current inspections of lap joints every 500 cycles on some 175 Boeing 737-300s, -400s and -500s. That will undoubtedly place strain on operators of the type, most of which fly outside the U.S. The requirements called for inspection within five days of receipt of the AD for airplanes that have accumulated 35,000 cycles or more, and within 20 days for airplanes that have flown between 30,000 and 35,000 cycles.

Now, Boeing recommends inspections at 30,000 cycles for a subset of airplanes manufactured between 1993 and 2000, at least until it finishes its analysis of the risks involved. The inspections apply only to models with a specific lap joint design no longer used in production, Boeing Commercial Airplanes vice president of marketing Randy Tinseth wrote in his blog, Randy’s Journal. As other airplanes with the same design approach 30,000 life cycles, they’ll also undergo the same inspections, he added.

Boeing has drawn praise for its prompt response to the incident from Southwest Airlines, and one might even credit the manufacturer for its candor in admitting it could not explain the reason for the fuselage failure in such a relatively young airplane. But for a company that for years has prided itself on its reputation for engineering excellence, a miscalculation of the magnitude evident in this case might well remove a bit of swagger from Boeing’s famously confident stride.